Compaction of finely divided metals



United States Patent 3,269,826 COMPACTION OF FINELY DIV IDED METALSWalter F. Bumgarner, Timonium, Md., assignor to E. I. du Pont de Nemoursand Company, Wilmington, Del., a corporation of Delaware No Drawing.Filed Oct. 8, 1963, Ser. No. 314,610 7 Claims. (Cl. 75--10) Thisinvention relates to the compaction or conversion of powdered refractorymetals into coherent masses and more particularly to novel methods andmeans for producing compacted masses of such metals.

As is known, refractory or high melting metals having atomic numbersbetween 21 and 79, including zirconium, titanium, tantalum, nickel,chromium, tungsten, cobalt, columbinum, hafnium, molybedenum, vanadium,etc., are recoverable in powdered or granular state. Thus, titanium isproduced in sponge form by reducing at an elevated temperature titaniumtetrachloride with a reducing metal such as magnesium or sodium, thesponge product being thereafter ground or milled to the particulatepowder or grain size desired. Also, columbium can be recovered in theform of free-flowing grains by reducing columbium pentach'loride withhydrogen at an elevated temperature; while nickel or tungsten powderscan be prepared by the reduction of their oxides in accordance withwell-known techniques.

The particulate products recovered from these operations are thenconverted to ingot form for fabrication into mill products, erg. bars,rods, tubes, strip, sheet or other useful article-s of commerce. Ingotfabrication is commonly brought about by a melting process, but manyrefractory metals as well as their alloys cannot be subjected to meltingwhile in contact with known crucible materials due to an undesiredimpurity contamination which ensues. To minimize or obviate this,melting by consumable electrode techniques is resorted to in which themetal undergoing melting is formed into an electrode which is then aremelted by an electric arc struck between the electrode and the ingot ora pool of the metal of the same composition within a cold crucible. Themolten metal thus comes in contact only with its own solid or compatiblephase frozen on the Walls of the crucible and formation and productionis assured of an uncontaminated ingot.

Many difficult problems attend these atempts at fabricating consumableelectrodes of the particulate refractory metals mentioned. For example,if the metal powders are compressed and sintered into the desiredelectrode shapes, an undesired atomospheric and container materialcontamination takes place at the sintering temperatures used. When merecold compaction is undertaken to avoid this contamination,disadvantageously, an undesirably low compact green strength ariseswhich renders the compact inadequate in many instances to withstand thehandling and mounting in the electrode holder to which it must besubjected. This is especially true in the case of columbium. A real needthus exists for an improved method designed to assure effective andsatisfactory compacting of particlulate refractory metals and theiralloys without attendant objectionable impurity contamination of themetals. A salient object of this invention therefore is to overcomethese and other disadvantages characterizing prior refractory metalcompacting and melting procedures. A principal object is to providenovel methods and means for attaining these objects, and particularly animproved method for compacting and melting particulate forms ofcolumbium and its alloys without the objectionable impuritycontamination alluded to. Other objects and advantages will be apparentfrom the ensuing description.

These objects are attained in this invention which comprises compactingparticles of a refractory metal, a refractory metal alloy, or mixturesthereof to a coherent mass by enclosing a charge of said particleswithin a container comprising a relatively thin foil or sheet of a metalcompatible in composition with said particles, which on completion ofthe compacting operation is adapted to function as a consumableelectrode during subsequent arc melting of the compacted mass,enveloping said container and its metal particle content within a thinsheet of high strength, solid organic polymeric plastic material,enclosing the resulting assembly in a flexible, airand watertightreceptacle, and subjecting the confined mass of particles while in saidreceptacle to hydrostatic compaction in liquid media.

In a more specific embodiment, the invention comprises compacting apowdered or granular refractory metal, such as columbium, to a coherentmass by enclosing a charge of the metal particles in a suitablecontainer element such as a thin foil or sheet material of columbiumwhich will function, on completion of the compacting operation, as anintegral part of a consumable electrode in the subsequent arc melting ofthe mass produced from the compacting operation, confining saidcontainer and its metal particles within and enclosing them completelyin a thin sheet of a high mo'lecular-weight, preferably thermoplasticorganic polymeric material such as polyethyleneterephthalate, enclosingthe said resulting particle mass and thermoplastic organic polymer sheetcontainer within a relatively thick flexible, airand water-tight rubberves- Sci, and subjecting the particles While confined in said containerwithin said vessel to hydrostatic compaction.

In practically adapting the invention particles of one or morerefractory metals having a melting point above l200 C., such astitanium, silicon, hafnium, vanadium, colurnbiurn, tantalum, tungsten,nickel, cobalt, etc. or an alloy thereof are compacted to a coherentmass by introducing a suitable charge of the metal or alloy particlesinto a relatively thin container comprising preferably a foil of metalor alloy of substantially the same chemical composition as the metalparticles ibeing subjected to compaction 'whereby the body of refractorymetal particles are completely enclosed in said foil. The closedcontainer is then Wrapped or otherwise enveloped in a thin plastic sheetor wrapper of a relatively high tensile strength, preferablythermoplastic organic polymer such as a p'olyethyleneterephthalate, orpolyvinyl acetate, polyvinyl alcohol, polyvinyl chloride, polyvinylfluoride, poly-vinylidene chloride, polyvlinylidene fluoride,polystyrene, polyurethane, or other terephthalic acid glycol polymers,polymethylmethacrylate, polyethylene, polypropylene, polybutylene,polyamides, chlorinated polyethers, polyacrylo-nitrile, polycarbonates,p olychlorotrifluoroethylene, polymeric fluorocanbons such aspolytetrarfluoroethylene, and also cellulose ethers and cellulose esterssuch as ethyl cellulose, cellulose acetate, cellulose propionate,cellulose acetate butyrate, cellulose nitrate, etc. If desired use canbe effected of copolymers based on two or more of the above materials.The commercial thermoplastic polymers utilizable in this invention aredescribed in the Plastics Properties Chart, Part 1, found in ModernPlastics Encyclopedia (Plastics Catalogue Corporation, Bristol,Connecticut, 1962). Preferred sheets are those composed of polyethylene,polyethyleneterephthalate, polyvinyl chloride, 6,6 nylon, and a sheet ofpolyethyleneterephthalate coated with polyethylene. The plastic sheet orfilm can range from 3-10 mils in thickness and is used to cover themetal foil enclosure at least to the extent that it separates said foilfrom contact with an outer flexible or rubber container to the presentlyreferred to. When it is desired to evacuate the package, provision forthe egress of air can he made. In general, the plastic films employedherein should have a tensile strength of 10,000 p.s.i. or higher andpreferably a tear strength of gms. or higher, based on the well-knownElmendorf test.

The resulting package is then enclosed within a relatively strong,flexible, airtight vessel or container which can comprise a heavyflexible sleeve or boot made of synthetic or natural rubber or similarsynthetic material. The equipment and procedure at this step of theprocess is well known in the practice of hydrostatic compaction. Thesleeve is usually closed at one end and made to fit the compact. Theopen end may be sealed by folding, tieing, or by applying suitableadhesives against the inflow of the pressurized liquid. Frequently, itis desira'ble to evacuate air from the compact which may be done byincorporating a check valve in the top enclosure with provision forconnection to a suitable vacuum pump.

The whole, lfinal assembly is then immersed in a suitable liquid mediumsuch as water, contained in a pressure vessel and subjected tosufficient hydrostatic pressure to eflFect desired compaction of theparticular metal. Sufiicient pressure is applied to compact theparticles and their associated metal foil container element and providethem with a substantial degree of adherence at least within the toil andthe particles contacting said foil. The pressure may be applied bymechanical means such as pumps or by explosive or spark discharge in theliquid in accordance with well-known procedures. Depending on the methodof applying the pressure, the time under pressure may vary upward from afraction of a sec-ond. The times and pressures used are selected on thebasis of the material and equipment employed and are, as noted, appliedin accordance with conlventional procedures.

After the desired compaction has been effected the entire package orassembly is removed from the pressure vessel and the outer flexiblecontainer or boot is removed from the compact and the sheet is peeledaway. Advantageously a polymeric thermoplastic organic sheet such ascommercial polyethyleneterephthalate film, which is preferred for use,does not adhere to the metal and can be completely removed therefromfollowing the compaction. The metal foil wrapper is left on the compact,forms an integral part thereof, and is subsequently consumed with theelectrode in the melting operation to which the compact is latersubjected. During said compaction the metal 'foil becomes pressedagainst and into the charge of metal particles to form as noted anintegral part thereof and therefore advantageously stabilizing thecompacted product. The foil being integrally associated with the chargerenders the whole compact rigid and strong enough for handling and useand in the subsequent arc melting of the compact the foil advantageouslyprovides a consumable electrode. In many instances where cohesion of theparticles is relatively poor, as with pure columbium and certaincolumbium alloys, the particles would run out of the compact if the[foil is deliberately broken. Nevertheless, as long as the toil is leftintact, the compact can be readily melted without spilling of solidmetal particles with the foil acting as a consumable electrode. This isrendered possible because in the process of striking the arc theelectrode becomes heated to sintering temperatures at least in thevicinity of the are. In a preferred melting operation, the electrodesare initially shorted to eifect the sintering condition prior to formingthe are for melting.

To a clearer understanding of the invention the following specificexamples are given which are merely illustrative of and not to beconstrued as limiting the underlying principles of the invention.

Example I A pure columbium sheet 11% x 28" x 4 mills was wrapped about a3%" diameter cylindrical wood form and lap seamed to form a tube. It wasslipped oif the end of the form and fitted with end caps made fromsimilar foil. The end caps were made from 5 /2" diameter discs byserrating and bending the edges to form caps or plugs which fittedinside the end of the tube. One end was closed with one of these caps toserve as the bottom of the container. A sheet of 8 mil commercialpolyethyleneterephthalate film known as Mylar, a trademark of E. I. duPont de Nemours and Company, was wrapped around the cylinder andfastened with thin adhesive tape. An end support consisting of a solidaluminum disc A" thick by 3%" diameter was placed against the lower endof the container and taped in place. This served .to keep the end of theresulting compact fiat to facilitate tack welding of several compactsinto a longer electrode if desired. The cylindrical container was thenfilled with pure commercial columbium granules of approximately 0.5 mm.diameter. The top foil cap was pressed into place, the amount ofgranules being adjusted so that when the cap was firmly in place it wasflush with the end of the cylinder. A second x 3% inch aluminum disc wasplaced on top. This assembly was then set into a fairly close fittingNeoprene synthetic rubber sleeve open at one end. The open end wasclosed by clamping. This whole package was disposed within a pressurevessel, the cover bolted on said vessel and the vessel was filled withwater and connected to a high pressure force pump. The hydrostaticpressure in the pressure vessel was raised to 60,000 p.s.i. andmaintained for two minutes. The pressure was let down, the vessel openedand the compact removed. The rubber sleeve was easily slipped off andwas suitable for re-use. The plastic film was closely immeshed in thefolds of the columbium foil but could be completely peeled off, leavingan uncontaminated columbium compact about 3 in diameter and 26" longadapted for use as the consumable electrode in a vacuum arc meltingfurnace. The furnace chamber contained a cooled copper crucible sectionin which the ingot was cast. To start the arc melting process a layer ofcolumbium granules was placed in the bottom of the crucible. Thecompacted electrode was clamped in place. The electrode was lowered tomake firm contact with the striker material in the crucible. The currentwas then turned on and increased to about 5000 amps. in about 5 minutesand held at 5000 amps. for one minute. The electrode was thus heated tosintering temperature. The current was then cut off and the electroderaised out of contact with the crucible. The are current was thenapplied, the arc struck and the electrode melted in the usual manner. Itmelted oif smoothly with no evidence that the solid granules werefalling out.

Example 11 Eight hollow cylinders of approximately 5 mil. columbium foil4 diameter x about 14" long Were prepared with end caps. A powdermixture was made up by blending 36.5 lbs. of 30+200 U.S. standard meshlow oxygen titanium, 16.0 lbs. of inch zirconium granules, and 278.0lbs. of 20+60 mesh columbium powder. A total of 5.5 lbs. of Cb foil wasused. The powder mixture was moistened with 1% isopropanol, mixed in acone blender, and transferred to the foil cylinders. The foil cylinder-swere then closed at the top with foil covers and wrapped in 10 mil highstrength polypropylene film. These packages were then placed in heavyrubber containers which were connected to a vacuum line and vacuumapplied for 1 hour to remove the residual isopropanol and entrapped air.The containers were closed and sealed, immersed in the water chamber ofa hydraulic press and subjected to a pressure of 60,000 p.s.i. for 5min. The assembly was opened and the film wrapped metal body was removedfrom the rubber containers and the film was stripped cleanly from themetal foil. Although the film was torn by this rem-oval it was cleanlyremoved without trouble. The metal compacts could be handled normallyand were 3,269,826 5 vacuum arc melted to 6 inch diameter ingots. Thesemetal being compacted. The commoner metals are obingots were joinedtogether and given a second arc melting to form an 8" ingot. The ingotcomposition was satisfactorily uniform and was substantially correct atthe desired 10% Ti, 5% Zr, 85% Ob. Approximately 8% of the titanium waslost by evaporation in the arc. The amount of titanium used wasincreased by this amount to compensate for this anticipated loss.

Example III 18 lbs. of 200 mesh (U.S. Standard Sieve Size) titanium, 2lbs. of30+325 mesh V-Al alloy (60% Al) were blended in a cone blender. Afour inch diameter aluminum foil cylinder was made and wrapped in a 5mil vinyl chloride-vinylidene chloride polymer sheet having a tensilestrength of 12,000 psi. to give support to the foil cylinder duringfilling. The top end was closed with both foil and film and the packageplaced in a heavy flexible neoprene boot and subjected to hydrauliccompaction as in Example II. The compacted metal was recovered cleanlyfrom the polymer wrappings and vacuum arc melted to an ingot. The ingotcomposition was 6% Al, 4% V. The aluminum foil used weighed about 0.25lb. but this amount of aluminum was lost by evaporation during thevacuum melting.

When using consumable electrodes, the foil used for the inner containeris melted and usually becomes part of the ingot composition. In view ofthis the covering metal sheet or foil chosen should be compatible withthe composition of the final ingot being produced, both in quantity andkind. The foil may have a relatively high vapor pressure andconsequently is more or less volatilized during the melting process. Aconsiderable portion of titanium foil, :for example, would be lost whenused during the melting of a columbium or tungsten alloy. Aluminum islost when used with a titanium alloy. This loss may be compensated (forby employing a corresponding excess of the volatile metal. It is usuallypreferable When economically feasible, to use foil of the metal formingthe major constitutent of the compact. Although aluminum foil willeffect a satisfactory compaction of columbium particles, most columbiummetal products are adversely effected by even small amounts of aluminum.Consequently use is preferred of a columbium foil where columbium orcolumbium base alloys are being compacted and melted although a foil cancomprise vanadium, titanium, etc. When formation is desired of alloyscontaining these or other elements. During the compaction the foil isfound to become indented between the metal particles, a condition whichno doubt contributes to the strength of the compact. In order towithstand this action the foil should be ductile. A good measure of thedesired ductility is that it be able to stand creasing, i.e. bending 180and then opening 90 iwithout cracking. The pure refractory metals andmany of their alloys will fulfill this requirement which makes itpossible to use lap seams in preparing the foil container. It also givesassurance that cracks will not open as the foil folds during compaction.Foil thickness of about 1 to 10 mils is satisfactory. The foil is notsealed in an air tight manner but merely folded with the ends cappedquite closely. The cracks left should be considerably narrower than thediameter of the adjacent particles.

It is sometimes advantageous to join several of the compacts to makelonger or continuously operating electrodes. To facilitate this it ishelpful to place end supports at each end of the foil container insidethe outer boot, which boot can consist of polymeric materials possessingelastic characteristics, such as natural or synthetic rubber, neopreneor hutyl rubber, etc. These sup ports are strong, relatively thick discsof some suitable metal such as brass, copper, aluminum, steel or of theviously preferred. These discs are relatively thick so as not to bedeformed by the compacting action. example, they may be A to 1 inchthick and having the diameter of the initial foil container or slightlyless. These end supports result in flat square ends easily tack welded.

The size of metal particles compactable under the invention is subjectto Wide variance. A practical size ranges from about 50 microns to /2inch in maximum diameter. The smaller particles provide smoothercompacts but are more subject to contamination, segregation, and siftingout of the wrappers. Very large particles present a rough contour whichmay damage the protecting films or even the boot and prevent theessential exclusion of the pressurizing liquid from the interior of thecompact.

The improvement of this invention resides in the combined use of a metalfoil enclosure covered by a strong plastic film. The foil in itself hasan advantage over heavier enclosures used in canning billets for workingin that it is impressed into the surface interstices of the metalparticles which aids in binding the assembly. The thin metal offers aminimum of resistance to the transfer of pressure and compaction of themetal particles themselves. In some cases, notably with columbium, thestrength of the compact is largely due to the foil wrapper. For example,in preparing a three inch columbium compact, the columbium particleswill flow out freely if the foil is opened, yet the electrode can behandled, clamped and tack welded without loss of particles. Aspreviously mentioned the electrode may be raised, by electricalresistance heating, to sintering temperature prior to actual melting sothat the solids do not fall from the compact.

If the foil is omitted, and the metal powder enclosed in plastic only,the result is unsatisfactory. In many instances the plastic rupturesduring compaction. It penetrates the interstices of the compact to theextent that it cannot be removed and hence causes contamination withcarbides etc. on melting.

The high strength plastic film used over the foil serves as a partingagent between the heavy outside, Water proof sleeve and the metal.Without the Mylar or similar lining the sleeve adheres to or becomestrapped in the wrinkles of the metal and cannot be thoroughly removed.Obviously contamination will result from this.

The manner of enclosing the metal particles in the foil and plastic filmmay be varied. The essential feature is to have the particle masssubstantially completely enclosed in a metal foil wrapper which isadjacent to and in contact with the peripheral particles. The plasticfilm is disposed externally of the foil covering, hence providing asubstantially complete separation between said foil covering and theouter or flexible container of the assembly. The heavy flexible outercontainer or boot encloses this double wrapped package against theinflow of the high pressure liquid. If desired, the whole triplexenclosure assembly made up of the foil, plastic film and the flexibleboot may be first assembled to form a container provided with an openingor an inlet in its top for admitting the desired particulate metalcharge. The foil and plastic film can be closed by folding or cappingthe foil and film in the order given and then closing the bootenclosure. Alternatively, and if desired, the foil, or the foil andplastic wrapper can be shaped into a container, filled with particulatemetal particles and the whole then be placed in the boot element. Inanother variation the exterior heavy flexible container may be appliedby dipping the inner assembly in a melt of suitable plastic material ora preparation such as one of the viscous compositions used to protect ormoth ball machinery.

Briefly the process of this invention has advantages ofnon-contamination and better compaction over previous methods ofpreparing metal compacts, especially those For destined for melting asconsumable electrodes. Many heretofore non-comp-actable metals may becompacted by this method and conveniently arc melted.

I claim:

1. A process for compacting a mass of metal particles which comprisescompletely confining said particles within a thin, 1-10 mils thick metalreceptacle enclosure, enveloping the latter within a thin, high strengthpolymeric plastic material, enclosing the resulting package Within astrong, flexible, liquid-tight container, and subjecting the entireassembly to hydrostatic pressure compaction.

2. A process for compacting a mass of metal particles which comprisescompletely confining said particles within a thin, 110 mils thick metalreceptacle enclosure, compatible in composition to said metal particles,enveloping the latter Within a thin, high strength polymeric plasticmaterial, enclosing the resulting package within a strong, flexible,liquid-tight container, and subjecting the entire assembly tohydrostatic pressure compaction.

3. A process for compacting a mass of metal particles which comprisescompletely confining said particles within a thin, 1-10 mils thick metalreceptacle enclosure compatible in composition to the metal particlesbeing subjected to treatment, enveloping said receptacle and itsparticles content within a thin, high strength polymeric plastic filmhaving a thickness ranging from 3-10 mils and a tensile strength of atleast 10,000 psi. enclosing the resulting package within a strong,flexible, liquid-tight container, and subjecting the entire assembly tohydrostatic pressure compaction.

4. A process for compacting a mass of columbium metal particles whichcomprises completely confining said particles within a thin, 1-10 milsthick sheet of columbium metal as an enclosure, enveloping saidenclosure within a thin, high strength polymeric plastic film, enclosingthe resulting package within a strong flexible, liquid-tight containerand subjecting the entire assembly obtained to hydrostatic pressurecompaction.

5. A process for compacting a mass of columbium metal particlescomprising completely confining said particles within a receptaclecomprising a thin, 1-10 mils thick columbium sheet, enclosing said sheetand metal particles within a thin, high strength polymeric plastic filmof from 3-10 mils thickness and having a tensile strength of at least10,000 p.s.i., confining the resulting package within a strong flexibleliquid-tight rubber container and subjecting the entire assembly tohydrostatic pressure compaction.

6. A process for compacting a mass of metal particles Which comprisescompletely confining said particles Within a thin, 1-10 mils thick metalfoil enclosure as a receptacle, enveloping said receptacle and its metalparticles content Within a polyethyleneterephthalate plastic film,enclosing the resulting package within a flexible, liquidtight rubbercontainer, and subjecting the entire assembly to hydrostatic pressurecompaction.

7. A process for compacting metal particles and melting the compactedproduct which comprises completely confining said metal particles withina thin-walled l-10 mils thick metal receptacle as an enclosure therefor,enveloping said receptacle within a thin, high strength polymericplastic material, enclosing the resulting package within a strong,flexible liquid-tight container, subjecting the resulting assembly tohydrostatic pressure compaction, recovering the resulting compactedproduct with said thin metal receptacle forming an integral part thereofand subjecting the recovered product to are melting.

References Cited by the Examiner UNITED STATES PATENTS 1,226,470 5/1917Coolidge 264-111 X 2,298,908 10/1942 Wentworth 264-111 X 2,648,1258/1953 McKenna et al. 264-111 X 2,783,504 3/1957 Hamjian et al. -214 X3,022,544 2/1962 Coursen et al. 264-111 X 3,023,462 3/1962 Taylor et al.264-111 X 3,041,660 7/1962 Fink 264-271 X 3,054,147 9/1962 Archibald264-111 X 3,112,166 11/1963 Montgomery et al. 264-111 X I-IYLAND BIZOT,Primary Examiner.

DAVID L. RECK, Examiner.

H. W. TARRING, Assistant Examiner.

7. A PROCESS FOR COMPACTING METAL PARTICLES AND MELTING THE COMPACTEDPRODUCT WHICH COMPRISES COMPLETELY CONFINING SAID METAL PARTICLES WITHINA THIN-WALLED 1-10 MILS THICK METAL RECEPTACLE AS AN ENCLOSURE THEREFOR,ENVELOPING SAID RECEPTACLE WITHIN A THIN, HIGH STRENGTH POLYMERICPLASTIC MATERIAL, ENCLOSING THE RESULTING PACKAGE WITHIN A STRONGFLEXIBLE LIQUID-TIGHT CONTAINER, SUBJECTING THE RESULTING ASSEMBLY TOHYDROATATIC PRESSURE COMPACSAID THIN METAL RECEPTACLE FORMING ANINTEGRAL PART THEREOF AND SUBJECTING THE RECOVERED PRODUCT TO ARCMELTING.